Actinomyces / Nocardia Infection

Infection with the bacterial organisms Actinomyces bovis and Nocardia asteroides is seen not infrequently in our practice. Most commonly we see these infections in field trial and hunting dogs. Summer conditioning of these dogs, especially those taken to Canada or the Dakota’s, seems to be associated with increased prevalence of infections. Both bacteria are found throughout nature, commonly in the soil (Nocardia), and in the oral cavity and bowel (Actino). Clinically, symptoms of infection with either organism cannot be differentiated and they will be discussed together. Route of infection appears to be either through penetration of skin, inhalation, or penetration of mucosal barriers after ingestion. Frequently, it appears the organism and infection is associated with contamination of a body cavity by a foreign body such as a grass awn (speargrass, foxtail). In the hunting dogs, it appears inhalation or ingestion of the grass awn are the primary modes of entrance, with subsequent migration of the awn to a body cavity. Several clinical syndromes of infection have been described; our focus will be on the two most commonly seen in our practice.

The thoracic form of infection occurs when a grass awn migrates via the lung tissue into the thoracic cavity. Dogs may present with a dry, non-productive cough, or simply be presented for exercise intolerance. Symptoms may be gradual in onset, especially in highly trained, athletic dogs. Chest radiographs may demonstrate fluid within the chest cavity (pyothorax) with a mass or masses within the lung lobe(s). Recently, I have seen two cases with a diffuse bronchointerstitial pneumonia rather than a mass lesion and pyothorax. Thoracocentesis in cases with pyothorax demonstrates a consistently foul-smelling, “tomato soup-like” fluid which is usually a greyish-yellow to reddish-brown in color. Granules may be noted in the fluid. When submitting cultures, be sure to notify the lab you are looking specifically for Actino/Nocardia infection as special culture media and a prolonged culture duration is required. Thoracotomy is usually indicated, with aggressive surgical debridement plus/minus lobectomy. Mortality rates of approximately 50% may be expected. Conservative treatment is not effective.

The cutaneous form is the other frequent manifestation of disease that we see. It may be characterized by a draining tract on the extremities, but more often we see a large abscess-type lesion on the thoracic wall, or just caudal to the last rib on the abdominal wall. Draining the abscess rarely results in a cure. Surgical exploration with aggressive debridement is needed. Identification of a foreign body markedly improves the odds of success, with success rates of 90% expected if a foreign body is found.

Recurrent infection, or persistent low-grade infection may be noted. Aggressive, long-term antimicrobial therapy is needed. Since the organisms are difficult to culture, treatment many times must be done in the absence of a definitive culture and sensitivity finding. Combination therapy with penicillin/amoxicillin plus sulfa-trimethroprim for one month is suggested. Alternatively, 11 mg/kg clindamycin BID, used for 30 days, is appropriate based upon recent communication with a veterinary microbiologist. Cost or side effects may dictate antibiotic choice.

Awareness, early detection, and aggressive treatment of these infections improves success rates dramatically. Hopefully, this will serve to heighten the suspicion and recognition of Actinomyces and Nocardia infection.